Microelectromechanical systems (MEMS) devices include semiconductor chips which include microfabricated mechanical systems on the chip, such as sensors.
For some applications, it is desirable to obtain real-time 2-D profiling of certain physical parameters such as temperature, force, pressure or shear stress on a 3-D object. If the surface of the object is flat, this profiling can be achieved by using a monolithic MEMS device with a large amount of sensors. However, such MEMS devices are typically rigid and flat. Profiling becomes much more difficult if the surface is not flat.
For example, in aerodynamics study, research objects such as an airfoil have non-planar and high-curvature surfaces. Previous attempts to achieve real-time distribution measurement embedded the discrete sensors on a surface. However, large sensor size and difficulty in packaging, i.e., plumbing and wiring, have long been limiting factors to realizing good measurements.
Barth et al. in 1985 reported a one-dimensional flexible Si-diode temperature sensor array in which a polyimide strip was used as a flexible material connecting Si islands formed by isotropic hydrofluoric, nitric, and acetic acid (“HNA”) etching. Here, polyimide refers to a polymer of imide compounds, those that contain the ═NH group. However, this technology needed some major improvements before it could be applied to more complicated sensor systems.
In 1994, Beebe and Denton presented their effort on improving the robustness and reliability of flexible polyimide skins which did not include any devices. The authors identified a major cause of failure as breaks in thin silicon on the island periphery. The methods used to enhance the robustness, including the application of tape and coating of epoxy on both front and back sides of the skins, were all performed manually as post-processing steps. These methods are not an ideal solution for a reliable as well as mass-producible smart skin technology.
Bang and Pan have an on-going project to develop a flexible heat-flux sensor array which is made by direct deposition of thin-film metals on commercial Kapton™ substrates. A large array of metal temperature sensors can be made in this way, but neither ICs nor silicon MEMS are easily integrated with this approach. Hence, only limited types of sensors are available using this approach and a hybrid assembly of electronic circuits is not readily avoidable.